Estimation of Microbial Activities in Lake Sediments by Measurement of Sediment Gas Evolution

The employment of an inverted sheet-metal cone permitted measurement of in situ evolution of methane and carbon dioxide from various Lake Erie sediments. The methane evolution rates varied from 0 to 71.5 mg/m2/h, while the carbon dioxide evolution rate reached a maximum of 6.1 mg/m2/h. The corresponding in vitro methane evolution or production rates demonstrated similar trends in some cases, but in other cases, obvious variations were observed. The in vitro variations in methanogenesis were attributed to the physical disruption of the sediment, which may have resulted in oxidation of the sample. The corresponding dissolved methane concentrations in the water column correlated well with the in situ methane evolution rates. In sediment regions where distinct lateral gradients of methanogenesis were observed, a corresponding gradient was observed in the overlying water column. Sediment regions consisting of clay, sand, or clay-sand mixtures showed no gaseous evolution. Minimal methane concentrations were observed in the water column above these sediments. The more active gaseous evolution occurred at near-shore regions, where external loadings contributed to the organic loading of the sediments. The use of this inverted cone proved to be critical in assaying in situ methanogenesis. The in vitro methanogenic rates were susceptible to oxidation and resulted in variable estimations of net sediment methanogenesis. The association of this sediment methane and carbon dioxide evolution with sediment denitrification rates (nitrogen evolution) could provide valuable data for carbon and nitrogen budgets in freshwater systems.